Revision as of 17:24, 28 August 2008

Contents

Abstract

This tutorial needs KDE 4.2 (trunk) to build. For correct behavior you should also build ExtenderApplet in playground (I really should be moving this to kdebase soon). And finally, you'll need to build the kuiserver dataengine (also in playground).

In this tutorial I will show how to create a simple applet that uses an extender. This applet is basically a very stripped down version of the kuiserver applet.

About extenders

Extenders allow applets to, with only very little extra lines of code, use relocatable widgets. These Plasma::ExtenderItem objects, can be dragged around by the user, dropped anywhere, and can even be persistent between sessions. They can also keep running, even when the applet that created them, isn't.
The class that shows and manages these extender items is Plasma::Extender.

How to approach extenders

If you want to use extenders in your applet you'll usually have to do a couple of things:

optional: Implement a initExtenderItem(ExtenderItem*) function in your applet, if you want to make your extender items persistent. Use this function to create a widget to display in your ExtenderItem.

optional: Add some extra actions to your ExtenderItems.

To don't have to border with your applet resizing correctly when the extender changes size, and to have your applet automatically inconified when in a panel, I'd strongy advice you to use PopupApplet as a base class for your applets if you wish to use extenders. It will spare you a lot of hassle.

The code

Here I will show you our very simple stripped down kuiserver applet. It will show every new job in an ExtenderItem using a Plasma::Meter to display the jobs progress. In the title bar of the ExtenderItem the name of the datasource will be displayed, and a remove ExtenderItem action is added, so a 'delete' icon is displayed in the items titlebar.
Finally, the items will also be sort of persistent. When plasma is restarted, all detached items will get shown again, but will only show the name of the original datasource it was connected to as title. Kuiserver datasources are not persistent between sessions (yet?) so the meter won't show progress anymore. Yes, it isn't very usable, but this is only an example.

//Implement this function to make ExtenderItems persistent. This function will get called on
//plasma start for every ExtenderItem that belonged to this applet, and is still around.
//Instantiate the widget to be wrapped in the ExtenderItem here.
void initExtenderItem(Plasma::ExtenderItem *item);

public slots:
//We want to add a new ExtenderItem everytime a new job is started.
void sourceAdded(const QString &source);

};

K_EXPORT_PLASMA_APPLET(extendertutorial, ExtenderTutorial)

endif

This header should be mostly self explanetory. We use PopupApplet to have a nice basis for this kind of applet, which requires us to implement graphicsWidget().

//An init() function like this is usefull for most applets that use extenders.

//Always instatiate an extender in init, and not earlier, since extenders need to access
//configuration, and a corona is needed for that.
new Plasma::Extender(this);
//The message to be shown when there are no ExtenderItems in this extender.
extender()->setEmptyExtenderMessage(i18n("no running jobs..."));
//A sane size policy: a fixed verticle size feels natural for vertical lists.
extender()->setSizePolicy(QSizePolicy::MinimumExpanding, QSizePolicy::Fixed);

//Create a layout, and add the extender to it.
QGraphicsLinearLayout *layout = new QGraphicsLinearLayout(this);
setLayout(layout);
layout->addItem(extender());

//Monitor the geometry of the Extender, so popupapplet can resize the applet when necesarry.
//I'm working on making this no longer necesarry in the future. Actually, I'll put it next on my todo
//list, just ignore this line for now.
connect(extender(), SIGNAL(geometryChanged()), this, SLOT(widgetGeometryChanged()));
//Notify ourself whenever a new job is created.
connect(dataEngine("kuiserver"), SIGNAL(sourceAdded(const QString &)),
this, SLOT(sourceAdded(const QString &)));

}

QGraphicsWidget *ExtenderTutorial::graphicsWidget()
{

//We want to have the Extender as 'popup widget'
return extender();

}

void ExtenderTutorial::initExtenderItem(Plasma::ExtenderItem *item)
{

//Create a Meter widget and wrap it in the ExtenderItem
Plasma::Meter *meter = new Plasma::Meter(item);

//Add a new ExtenderItem
Plasma::ExtenderItem *item = new Plasma::ExtenderItem(extender());
initExtenderItem(item);

//We give this item a name, which we don't use in this example, but allows us to look up
//extenderItems by calling extenderItem(name). That function is useful to avoid duplicating
//detached ExtenderItems between session, because you can check if a certain item allready
//exists.
item->setName(source);

//And we give this item a title. It's stupid, but it shows how to make your items persistent
//between sessions.
item->setTitle(source);
item->config().writeEntry("title", source);

//Connect a dataengine. If this applet would display data where datasources would have unique
//names, even between sessions, you would want to do this in initExtenderItem, so that after a
//plasma restart, datasources would still get connected to the appropriate sources. Kuiserver
//jobs are not persistent however, so we connect them here.
dataEngine("kuiserver")->connectSource(source, item->widget(), 200);

//Show the popup for 5 seconds if in panel, so the user notices that there's a new job running.
showPopup(5000);

}

include "extendertutorial.moc"

This piece of code is well documented so I won't explain this any further.

The rest

Finally you'll need a cmakelist, a desktop file, and a svg for the meter widget. You can download those here:

TODO: upload files here.

So there it is, all it takes to make a simple applet that uses an extender. For a more complete example I'd like to point you at the kuiserver applet which is in playground right now. And I'd encourage you to read to api documentation of Extender and ExtenderItem. I'm looking forward to see your extender using applet, be creative! :)